Method of manufacturing stria-free, bubble-free and homogeneous quartz-glass plates

ABSTRACT

A method of manufacturing practically stria-free, bubble-free, and homogeneous quartz-glass plates of any desired configuration and with a surface area that exceeds the cross-section of the full circular quartz-glass cylinder that is employed as a starting material. The cylinder is continuously lowered into a furnace shell flooded with an inert gas, in which it is heated to a flowing temperature in the range of 1700° to 1900° C. until some of the quartz-glass flows off into a graphite crucible. The crucible is preferably clad with zirconium-oxide.

This is a continuation of application Ser. No. 501,478, filed June 6,1983, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a method of manufacturing practically stria-free,bubble-free, and homogeneous quartz-glass plates of any desiredconfiguration from a virtually stria-free, bubble-free and homogeneousfull quartz-glass cylinder as well as a device for carrying out themethod.

Virtually stria-free, bubble-free, and homogeneous quartz-glass shouldbe understood to mean glass that exhibits no or only very few striaswhen its transparent surfaces are tested with test equipment, that has amean overall bubble cross-section of less than 0.1 mm² /100 cm³ (BubbleClasses 1 and/or 0), and that exhibits a Δ n lower than 2×10⁻⁶ at a testaperture of 70 mm when tested with an interferometer.

2. Discussion of the Prior Art

A method of manufacturing planar article from molten quartz withelectric resistance furnaces is known from German Pat. No. 445 763. Inthis method, a rod-shaped electric heating resistance is surrounded withthe starting material to be melted. The current of heat produces atubular blank, from which then the resistance is immediately withdrawn,and thereafter the blank which is still soft is compressed into aplate-shaped body.

The manufacture of bubble-free quartz-glass articles is known fromGerman Pat. No. 697 699. The most frequent starting material is clearpieces of quartz obtained from rock crystal that are melted and thenshaped conventionally. The manufacture of quartz-glass articles bycold-manufacturing articles of the desired shape from silicic-acidpowder and then heating them in an electric furnace under vacuum untilthey become virtreous is also known from that document. The manufactureof blocks of quartz from granular material under vacuum that aresubsequently reheated and compressed into their final form is alsoknown.

Although the aforementioned methods of manufacture result in productsthat still contain bubbles, even when the products are specified to bebubble-free in German Pat. No. 697 699, U.S. Pat. No. 2,726,487discloses manufacture of essentially bubble-free, that is ratherlow-bubble content, clear quartz products by impregnating a granularstarting material such as pure quartz sand with a silicate solution anddrying the liquid components until SiO₂ precipitates in the pores of thestarting material, which is then melted down under vacuum in a graphitecrucible. The result is transparent planar bodies of quartz-glass withonly few bubbles.

A method of manufacturing plate-shaped bodies of quartz is know fromGerman Pat. No. 549 083. The material to be melted, which consists offine moist quartz sand, is placed in a flat cylindrical recess in asilicon-carbide stone. The material is then melted with flames from acurrent of heating gas that emerges from a hood positioned above it. Thequartz sand that is placed in the cylindrical recess (to a height of 8cm) is melted only part of the way through and the plate of quartz,which is 3 cm thick in the center and 2.5 cm thick at the edge, obtainedin this way is still positioned on fine quartz sand. The heat that isfocused on the material to be melted from above diffuses in alldirections because the silicon carbide stones are good heat conductors,meaning that the heat flows off laterally through the stones.

The manufacture of stria-free, bubble-free, and homogeneous quartz-glassplates of any desired given configuration by sawing a plate off apractically stria-free, bubble-free, and homogeneous full quartz-glasscylinder with a circular cross-section and then cutting the resultinground plate to the desired configuration, rectangular for example, isalso known. This method is of course restricted to the manufacture ofquartz-glass plates with surface areas that are smaller than thecircular cross-section of the cylinder.

The objective of the present invention is accordingly a method ofmanufacturing quartz-glass plate from a full quartz-glass cylinder bywhich the surface area of the resulting large plate may be of anydesired shape and essentially larger than the cross-section of thecylinder.

The invention achieves this objective in accordance with a methodwherein for the manufacture of large quartz-glass plates with a surfacearea that is essentially larger than the cross-section of the fullcylinder at right angle to the axis of the cylinder, the free end of afull quartz-glass cylinder is lowered vertically into a graphitecrucible, preferably clad with zirconium oxide, positioned inside ashell flooded with an inert gas, the cylinder is heated to aflow-temperature ranging from 1700° to 1900° C. and, when thistemperature has been attained, further lowered until the quartz-glassflows off into contact with the bottom of the crucible the cylinder isheated until the level of the quartz-glass that has flowed off into thecrucible equals the thickness of the desired plate and in that, coolingquartz-glass that has flowed off said quartz-glass cylinder, removingthe remainder of the full cylinder which has not flowed off and removingthe cooled quartz-glass which has flowed off the cylinder from thecrucible in the form of a plate.

The crucible can be of any size or shape. Hence, the bottom can have anyconfiguration or as result of which the stria-free, bubble-free,homogeneous quartz-glass plate can have any desired shape. The processalso allows infinite selection of the plate's thickness.

The process is preferably performed by initially evacuating the shell,thereafter heating the same to 1500° to 1700° C. before the shell isflooded with inert gas, which is preferably nitrogen.

Preferably the quartz-glass cylinder from which the quartz-glass isflowed off is lowered into the cylinder through an electric heaterpositioned above the graphite crucible in the shell. The fullquartz-glass cylinder is introduced into the crucible at a rate suchthat 40-60 kg of quartz-glass flow off of the cylinder per hour.Generally, the full quartz cylinder is lowered into position, heated toits flowing temperature and maintained at that position for at least 0.5hour.

The method in accordance with the invention permits for the first timethe manufacture of quartz-glass plates of an especially high opticalquality in any desired configuration and with dimensions thatconsiderably exceed those of the full-quartz cylinder employed as astarting material. It must also be emphasized that the flow-off processemployed in accordance with the invention considerably improves thehomogeneity of the starting material while the other materialproperties, freedom from stria and bubbles, match those of the startingmaterial and are retained. Furthermore, any stria already present in thestarting material are more uniformly distributed and the bubble contentis decreased as a result of diffusion. Quartz-glass plates manufacturedby the method in accordance with the invention have turned out to be notonly practically stria-free but also free of bubbles and to exhibit ahomogeneity of better than Δn=8×10⁻⁶ (at a test aperture of 700 mm) fora starting-material Δn of 1×10⁻⁵ (at a test aperture of 230 mm).

BRIEF DESCRIPTION OF DRAWING

The left side of the single schematic FIGURE shows the position of thefull quartz-glass cylinder during the heating-up phase and the righthalf of the position of the cylinder toward the end of the flow-offprocess.

DESCRIPTION OF SPECIFIC EMBODIMENT

Referring to the drawing, a water-cooled furnace shell 1 consists of abottom 2 and a top 3, which are connected vacuum tight. Shell top 3 hasa flange connection 4 that can be closed vacuum-tight with a cover 5. Aconnection 6 is positioned at shell bottom 2 and can be connected to avacuum pump or inert gas container such as a nitrogen bottle. Inside theshell is a charging table 7 that is preferably cooled.

A graphite supporting plate 9, covered with a layer 10 of carbon foam orfelt, is positioned on the surface 8 of charging table 7. A graphitecrucible, consisting of a bottom plate 12 and wall plates 13, is builtonto layer 10. Bottom plate 12 is covered with a layer 26 of zirconiumoxide that consists of a granular coating and of a coating of felt overthe granulate. Wall plates 13 are covered with fiber plates 27 ofzirconium oxide. A graphite heating plate 14 with an access opening 15is positioned on the upper edge of the crucible. A graphite sleeve 16extends up from access opening 15. The outside of the walls of thecrucible, the graphite heating plate, and the graphite sleeve is coveredwith carbon-foam or carbon-felt insulating bodies 17. Graphite heatingplate 14 is supplied with alternating current through connection 6 overelectric-power supply lines 18a and 18b from a source that is notillustrated.

The method of manufacturing quartz-glass plates with the device justdescribed will now be specified.

Flanged connection 19 in shell top 3 is closed, shell 1 evacuatedthrough connection 6, preferably to a pressure of approximately 1 mbar,and graphite heating plate 14 supplied with current, heating the shellto a temperature in the range of 1500° to 1700° C. When the pressure isabout 1 mbar, the vacuum pump is turned off and the shell flooded withnitrogen at normal atmospheric pressure. Cover 5 is is removed fromflanged connection 4, and a full quartz-glass cylinder 20 lowered intothe shell to the position shown on the left of the FIGURE. In thisposition, the free end of cylinder 20 is heated to a flowing temperaturein the range of 1700° to 1900° C. and, once this temperature isattained, lowered again until it comes into contact with the floor ofthe graphite crucible so that the quartz-glass will flow off. Cylinder20 continues to be heated until the level of the quartz-glass that hasflowed off into the crucible equals the thickness of the desired plate.Graphite sleeve 16 is used to preheat the cylinder in such a way thatits temperature will increase from top to bottom. The cylinder isintroduced at such a rate and the flowing temperature maintained suchthat about 40-60 kg of quartz-glass will flow off per hour.

Cylinder 20 is fused with a pipe component 22 attached to a suspensiondevice 23 that is preferably cooled and that is attached to a steelcable wound over a deflection pulley 25 on a capstan that is notillustrated. As soon as the level of the quartz-glass that has flowedoff into the crucible equals the thickness of the desired plate, whichwill occur once the cylinder has been lowered into the positionillustrated on the right of the FIGURE, the lowering of the cylinder isstopped and the quartz-glass that flowed off is maintained at theflowing temperature for at least 0.5 hours. The quartz-glass that hasflowed off is then cooled and the remainder of the full cylinder isseparated from the quartz-glass that has flowed off, which is thenremoved from the crucible in the form of a plate.

Full quartz-glass cylinder 20 is lowered into the graphite crucible witha capstan that is not illustrated and that is controlled in such a waythat the constant decrease in weight can be monitored with a tensiondynamometer 11.

Interrupting the cooling of the quartz-glass that has flowed off after atemperature of approximately 1100° C. has been attained to subject theglass to tempering the graphite heating plate 14 before it cools offcompletely is practical and desirable, in some instances.

The method in accordance with the invention has been successfullyemployed to manufacture quartz-glass plates measuring 1000×1000 mm andwith a thickness of 80 mm from a full quartz-glass cylinder with adiameter of 200 mm and a height of 3000 mm.

The invention is naturally not restricted to full quartz-glass cylindersmanufactured from natural quartz crystals but is also appropriate whenemployed similarly with full cylinders manufactured from syntheticquartz-glass.

What is claimed is:
 1. A process for manufacturing virtually stria-free,bubble-free, and homogeneous quartz-glass plates of any desiredconfiguration from a virtually stria-free, bubble-free and homogeneoussolid quartz-glass rod, which method comprises suspending a free end ofa solid quartz-glass rod, lowering said quartz-glass rod verticallythrough an opening in an electrical resistance heating means into agraphite crucible of a given configuration, said crucible beingpositioned inside a shell flooded with an inert gas, as said rod isbeing lowered through said opening, preheating the rod by a graphitesleeve extending from the crucible, said sleeve disposed around the rodand being a part of the electrical resistance heating means, saidpreheating being such that the temperature is increased from top tobottom of the rod, heating said rod while inside said crucible, to aflowing temperature ranging from 1700° to 1900° C. and after said rodhas attained such temperature, lowering the rod again until thequartz-glass flows off said rod directly into contact with the bottom ofthe crucible, continuing the heating of said rod until the level of thequartz-glass that has flowed off said rod into the crucible equals thethickness of the desired plate, cooling the quartz-glass that has flowedoff, separating the remainder of the solid rod from the quartz-glassthat has flowed off, and recovering the cooled quartz-glass from saidcrucible in the form of a plate.
 2. A process according to claim 1,wherein the shell is evacuated and heated to a temperature in the rangeof 1500° to 1700° C. before it is flooded with an inert glass.
 3. Aprocess according to claim 1, wherein the shell is flooded with nitrogenand a nitrogen atmosphere of normal pressure maintained in it.
 4. Aprocess according to claim 1, wherein the full quartz-glass cylinder isintroduced at such a rate and the flowing temperature maintained suchthat about 40-60 kg of quartz-glass flows off per hour.
 5. A processaccording to claim 1, wherein the lowering of the cylinder is stoppedand the quartz-glass that has flowed off is maintained at the flowingtemperature for at least 0.5 hours.
 6. A process according to claim 1,wherein a graphite crucible clad with zirconium oxide is employed.